1. Field of the Invention
This invention relates generally to apparatus and methods for removing ophthalmic lenses and more specifically for removing a cataractous ophthalmic lens for vision restoration.
2. Discussion of the Prior Art
The lens of a human eye is a crystalline, transparent biconvex intraocular tissue that helps bring rays of light to focus on the retina. The lens is enclosed in a lens capsule and consists of lens cortex, and lens nucleus. The lens capsule is an elastic bag enveloping the lens and is suspended by fine ligaments (zonule) attached to the ciliary muscles. These muscles radially stretch and relax the capsule thereby varying the optical characteristics of the enclosed lens to provide the desired focus for an image. This is commonly referred to as accommodation.
The lens cortex is a jelly-like portion of the crystalline lens, composed of a multiplicity of thin lens fibers that form the main body of the lens. The lens cortex is located between the denser inner nucleus and the elastic outer capsule. The lens nucleus is an optically defined-zone which is denser in the central position of the lens. This nucleus becomes even denser with age, eventually hardening and filling the entire lens. Additionally the lens may become opacified.
This opacity and cloudiness of the crystalline lens or its surrounding transparent membrane, commonly referred to as a cataract, may be congenital or may be caused by trauma, disease, or age. The cataractous lens obstructs the passage of light and tends to prevent the formation of a clear image on the retina.
Surgery currently is the only method of restoring vision in a patient blinded by cataracts. The surgical removal of the opacified lens becomes necessary when visual loss due to cataract becomes significant. The lost optical power is restored by a contact lens, aphakic spectacle, or intraocular lens.
The cataract has become one of the most significant and common causes of ocular disability and blindness in our aging population. Cataract procedure is currently the most frequent surgery performed for a person over the age of 65. There were 4 million (U.S.: 1.6 million; foreign: 2.4 million) cataract surgeries performed in 1991, a number which is growing at an annual rate of 5%.
The classic method of cataract surgery is the removal of the intact lens through a 7-10 mm incision and its replacement with an intraocular lens made from bio-compatible polymers. This extracapsular cataract procedure restores vision but often causes post-operative complications resulting from the large incision, which include a prolonged healing process, increased trauma and astigmatism. Nevertheless, a majority of the current cataract procedures in the U.S. are performed using this intact extracapsular cataract removal technique.
More recently phacoemulsification devices, relying upon ultrasound, have been used for emulsifying the lens and removing it through a 3 millimeter incision in a shorter operative time. This technique provides easier rehabilitation and eliminates most of the post-operative complications resulting from the larger incision of conventional extracapsular cataract procedures.
For the phacoemulslification procedure, a 3 mm limbal incision is made about 45.degree. to the iris plane allowing insertion of the instrument's tip into the anterior chamber in a direction almost parallel to the iris. Once the limbal incision has been made, the central part of the anterior capsule must be widely opened to facilitate emulsification of the lens nucleus and cortical clean-up, as well as to provide for an ideal intraocular lens placement in the sulcus of the posterior chamber.
Phacoemulsification can be performed in the anterior chamber or posterior chamber of the eye. In the case of anterior chamber phacoemulsification, the cataract lens is maneuvered into the anterior chamber where it is carved and removed from the chamber. This method is more traumatic to the endothelial layer of the cornea; however, it is an easier procedure for the surgeons to perform. Posterior chamber phacoemulsification consists of carving or shaving the central part of the lens while the lens is still in the capsule. This method is more difficult to perform due to the possibility of rupturing the posterior lens capsule and exposing the vitreous which fills the volume of the inner eyeball.
The phacoemulsification technique provides the advantages of a smaller incision, a stronger post operative globe which reduces astigmatism, better wound closure, lower trauma and earlier improvement in vision. However, the phacoemulsification procedure is contraindicated for patients with dislocated cataract lens, a shallow anterior chamber, miotic pupils, low cornea-endothelial cell counts, or myope (a totally hard lens). The phacoemulsification technique also requires intense training in maneuvering the ultrasonic probe to carve the cataract lens nucleus. The energy can be destructive to the endothelial cells of the cornea ultimately resulting in complete degeneration. Due to these adverse circumstances, only about 45% of the U.S. surgeons currently prefer to use this phacoemulsification method over the conventional extracapsular method for cataract removal.
Use of phacoemulsification devices to perform endocapsular cataract removal has also been investigated. In such a procedure, the cataractous lens must be carved away while both the anterior and posterior sides of the capsule are left intact. The extreme difficulty associated with this procedure has limited its adoption so that only about 1% of the U.S. cataract removal procedures are performed using this endocapsular technique.
Currently, there remains a need for apparatus that provide for safe and effective endocapsular lens removal, and associated methods which are less time consuming and skill intensive.